lt's fast to travel by underground.lt's safe to travel by underground as well.(合并成一句)lt's ______ _______ fast ______ ______ safe to travel by underground.

lt's fast to travel by underground.lt's safe to travel by underground as well.(合并成一句)lt's ______ _______ fast ______ ______ safe to travel by underground.
lt's fast to travel by underground.lt's safe to travel by underground as well.(合并成一句)
lt's ______ _______ fast ______ ______ safe to travel by underground.

lt's fast to travel by underground.lt's safe to travel by underground as well.(合并成一句)lt's ______ _______ fast ______ ______ safe to travel by underground.
not only but also

not only but also .原句翻译为：乘地铁出行很快，也很安全。as well as 有也的意思。所以合成一句的话就是：不仅快而且安全。not only but also 有不但而且的意思。